Means for supporting core in open ended shell mold

ABSTRACT

An open ended refractory shell mold having a casting cavity therein, a chill block positioned to close off the open end of said mold and a ceramic core having one end received within said mold and having a free end extending into said molding cavity. The free end of said core is restrained from lateral movement within said molding cavity by engaged pin means having opposed ends anchored in the mold walls.

O United States Patent 1151 3,659,645 Rose Ma 2 1972 [s41 MEANS FORSUPPORTING CORE IN 1,867,862 7/1932 Moore ..164/366 OPEN ENDED SHELLMOLD 2,362,745 11/1944 Davidson ..l64/363 2,835,007 5/1958 l-loefer..164/363 [72] Cleveland Ohm 3,204,303 9/1965 Chandley ..l64/361 [73]Assignee: TRW Inc., Cleveland, Ohio Primary Examiner-Robert D. Baldwin[22] 1965 Attorney-Hill, Sherman, Meroni, Gross & Simpson [21] Appl.No.1 478,080

[57] ABSTRACT 52 U.S. c1 "164/353, 164/361, 164/366 An open endedrefractory she mold having a casting cavity [51] Int. Cl. ,.B22c 9/10therein a chill block posiioned to close ff the open end of [58] new ofSearch "164/3611 said mold and a ceramic core having one end receivedwithin 164/368 249/177 said mold and having a free end extending intosaid molding f Ct d cavity. The free end of said core is restrained fromlateral [56] Re erences I e movement within said molding cavity byengaged pin means UNITED STATES PATENTS having opposed ends anchored inthe mold walls.

3,401,738 9/1968 Parille 164/361 X 6 Claims, 4 Drawing Figures \jjl b;\\Q Q l Patented May 2, 1972 MEANS FOR SUPPORTING CORE IN OPEN ENDEDSHELL MOLD The present invention relates to apparatus for the casting ofhigh temperature alloys and, more particularly, to the production ofcastings having controlled grain structure and exceptional soundness. Inthe preferred embodiment of the present invention, the apparatus is usedto produce columnar castings which have been found to be particularlydesirable in articles such as jet engine blades and vanes which aresubject in use to extreme heat and thermal cycling.

Recent work on columnar structures indicates that for some applications,such structures are markedly superior to equiaxed structures. Forexample, it has been found that the high temperature properties ofcolumnar structures are superior, particularly in fracture resistanceand ductility under creep loading conditions.

Columnar structures are formed by the unidirectional growth of dendritesduring solidification. The relationship between the dendritic structureand the columnar grains is not exact. Each columnar grain is usuallycomposed of more than one dendrite, and the number may vary from a fewto several hundred. The interdendritic spacing is related to thesolidification rate only. Columnar grain size, however, may be affectedby factors other than the solidification process such as ordinary graingrowth. Despite these differences, the most convenient approach for theexamination of columnar structure formation is through the study ofdendrites formed during solidification.

The primary requirement for the formation of a parallel dendriticstructure is the presence of a unidirectional thermal gradient duringinitial pouring of the metal into the mold, and during the controlledcooling of the casing during solidification. When the metal first entersthe mold, the initial solidification occurs at the mold wall due to achill effect, assuming the mold wall to be below the solidificationtemperature of the metal. This chill zone consists of many finedendrites having a random orientation. The initial freezing releases theheat of fusion, resulting in some temperature rise locally, arrestingthe chill zone formation. At the interface of the chill zone and themelt, the dendrites begin to grow into the melt at a rate dependent uponthe amount and depth of the supercooling.

Initially, all dendrites at the chill zone-melt interface grow at equalrates, since equal supercooling is present. However, those orientedparallel to the thermal gradient are growing into an area of continuedsupercooling. Those oriented unfavorably cannot advance as rapidly inthe direction of the thermal gradient, since only a component of thegrowth velocity is aligned with this gradient. The dendrites growingparallel to the gradient, since they have already undergrown somegrowth, will give off a latent heat of fusion, due to the freezingprocess. This heat of fusion increases the temperature at the base ofthe dendrites and decreases the amount of su' percooling available forgrowth of the more unfavorably oriented neighbors. In this manner, thegrowth of the misoriented dendrites is stifled, and only those alignedwith the thermal gradient will undergo significant growth.

The most suitable process for the production of columnar structuresemploys an open ended ceramic type mold of the shell mold type producedby conventional precision investment mold making processes. The open endof the mold is placed against a chill block consisting of a metal ofhigh thermal conductivity such as copper, and the ceramic shell mold ispreheated before pouring of the metal to provide a desired temperaturegradient. Upon pouring of the metal, the molten metal first contacts therelatively cold chill block and begins to solidify with the formation ofthe dendritic structures. By controlling the thermal gradient existingin the mold during pouring and afterwards, the major axis of thecolumnar grains can be preselected, and the grains grown with a desireddirectional orientation.

Cooling of the metal to solidification is equally important withadequate preheating. In the best technique thus far developed, heat iscontinually supplied to the mold during solidification of the metal toprovide a controlled cooling rate which is lower than that which wouldoccur if the source of heat were removed entirely. This controlledcooling can be achieved in several ways, one being a controlledde-energization of the heating source about the mold in order toestablish zones of differing temperatures. Another consists inphysically moving the casting during solidification relative to the heatsource for the mold, thereby maintaining desired thermal gradientswithin the body of metal remaining to be solidified.

While the techniques of columnar casting have been reasonably welldeveloped for completely solid castings, the use of this technique inconjunction with molds having cores therein has posed additionalproblems. Typically, a core used, for example, in the manufacture of aturbine blade having a hollow vane portion is composed of a ceramicwhich expands more than the material of the mold. It is accordinglydifficult to position such a core accurately within the molding cavity,particularly since it has to be supported in the molding cavity in adepending position, with a free end extending in relatively closeproximity to the highly thermally conductive chill block. Consequently,the core cannot be held in the conventional manner at the bottom of themold because of the chill contact requirements, yet it must bestructurally supported within the mold to assure proper location of thecored passage in the casting.

One of the objects of the present invention is to provide an improvedapparatus for casting, particularly for the growth of columnar grains,which provides means for supporting a core within a casting cavity in anopen ended mold.

Another object of the invention is to provide a means for supporting acore in a casting cavity against lateral displacement, withoutinterfering with grain growth or orientation during the casting andsolidification phases.

Still another object of the invention is to provide a method forachieving columnar grain structures in cored parts without interferingwith the normal columnar grain development.

A further description of the present invention will be made inconjunction with the attached sheet of drawings in which:

FIG. 1 is a view partly in elevation and partly in cross-sectionillustrating a wax pattern embodying a ceramic core, and used for theproduction of the shell mold of the present inventron;

FIG. 2 is a cross-sectional view of the finished mold assembly disposedon a chill block, and having a core supported therein in properalignment with the casting cavity;

FIG. 3 is a cross-sectional view taken substantially along the lineIII-III of FIG. 2; and

FIG. 4 is a view similar to FIG. 3, but illustrating a somewhat modifiedform of the present invention.

AS SHOWN IN THE DRAWINGS In FIG. 1, reference numeral 10 indicatesgenerally a disposable pattern composed of a material such as wax or thelike used for the manufacture of a cored turbine vane. The particularvane shown in the drawings includes a shroud portion 11, an arcuate vaneportion 12 and a root portion 13. A ceramic core 14 having a core printportion 16 is embedded in the wax during the formation of the pattern,to provide the cored passage in the finished turbine vane. The lower endof the core 14 is held in position by means of a pair of ceramic pins 17and 18 (FIG. 3) which abut the opposed sides of the core 14, and areprepositioned in the die in which the wax pattern is originally made.The ends of the pins 17 and 18 extend beyond the external limits of theroot portion 13 so that they can be ultimately received in the shellmold itself, as illustrated in FIG. 3 of the drawings.

While many different techniques can be used for producing the shell moldfrom the type of pattern shown in FIG. 1, I prefer to use a method ofmaking refractory molds described in U.S. Pat. No. 2,932,864. In theprocess described in that patent, the temperature of the patternmaterial is held substantially constant from the time the pattern isfonned until the pattern is removed from the shell mold. Typically, thetemperature of the pattern during this process may range from about 70to 80 F.

Initially, the pattern at room temperature or so is dipped in an aqueousceramic slurry having-a temperature about the same as that of thepattern material to form a refractory layer of a few mils in thickness.A typical slurry may contain ceramic materials such as zirconium oxide,and a binder such as methyl cellulose. The initial layer while still wetcan then be dusted with small particles (40 to 200 mesh) of a refractoryglass composition such as that known as Vycor which is a finely dividedhigh silicon oxide glass containing about 96 percent silica and a smallamount of boric acid together with traces of aluminum, sodium, iron, andarsenic. The dusted wet refractory layer on the pattern is thensuspended on a conveyor and moved through a drying oven having acontrolled humidity and temperature, so the coated pattern is driedadiabatically. When using air at a wet bulb temperature of 75 F., theprime coat can safely be dried by air having a relative humidity of 45to 55 percent.

The steps of dipping, dusting, and adiabatic drying are then repeatedusing air at progressively lower humidities for succeeding coats. Forexample, the first two coats can be dried with air having a relativehumidity of 45 to 55 percent. The third and fourth coats can be dried ata relative humidity of 35 to 45 percent, the fifth and sixth coats at arelative humidity of 25 to 30 percent, and the seventh and final coatwith a relative humidity of to 25 percent.

The first layer is preferably applied to a thickness of about 0.005 to0.020 inches, and the fine refractory particles are dusted onto the wetlayer with sufficient force to embed the particles therein. It ispreferred that the dusting procedure used provide a dense uniform cloudof fine particles that strike the wet coating with substantial impactforce. The force should not be so great, however, as to break or knockolf the wet prime layer from the pattern. This process is repeateduntila plurality of integrated layers is obtained, the thickness of thelayers each being about 0.005 to 0.020 inches.

, After the mold is built up on the pattern material, the pattern can beremoved by placing the same in conventional steam autoclave, and thenthe green mold is ready for firing.

Generally, firing temperatures on the order of l,500 to 1,900 F. areused. The resulting shell mold, identified at reference numeral in FIG.2, is hard, smooth, and relatively permeable, and measures on the orderof one-eighth to one-fourth inch in thickness.

The ceramic mold 20 includes acasting cavity 19 therein, and an openended portion 21 which is set on a chill block consisting of a block ofcopper 22 or the like. Heat transfer from the chill block 22 may befurther improved by circulating a suitable coolant through the block 22.Metal is introduced into the casting cavity 19 by means of a riser 23fed from a gate 24. While the core 14 may be dependently supported fromthe top of the shell mold 20 by means of the core print 16, it is notnecessary to do so in the case of the present invention. Accordingly,the core print 16 may be initially coated with wax during the formationof the pattern, so that there is a slight clearance space providedbetween the core print 16 and the material of the shell mold immediatelyadjacent it. The actual support function is provided by the pair of pins17 and 18 which have their ends embedded in the shell mold 20 asillustrated in FIG. 3 of the drawings. The pins 17 and 18 firmly holdthe free end of the core 14 in position as well as supporting itvertically. To further insure proper location, the pins can be firmlyattached to the core 14 by means of a ceramic adhesive paste.

A modified form of the present invention is shown in FIG. 4 wherein thetwo pins 17 and 18 are replaced by a single pin 26 which extends througha suitable hole drilled in the core 14 to provide a tight fittingengagement therewith. The ends of the pins 26a and 26b are firmlysecured within the shell mold to anchor the same and thus restrainlateral and vertical movement of the core 14. Then, upon pouring of themetal in the casting cavit l9, directional solidification resulting inthe production 0 the columnar casting can proceed without interferencewith respect to directional grain growth.

The ceramic core positioning pins shown in the drawings are located in aportion of the part which is subsequently cut or machined from thefinished configuration, so that it does not provide a defect in thefinished casting.

From the foregoing, it will be understood that the use of the coreholding pins of the present invention provides a highly effective meansfor locating the free end of a depending core portion within'an openended, ceramic mold. The pins are small enough so that they do notinterrupt grain growth, and the columnar grain grows around them. Still,they are strong enough to maintain the core in its proper positionwithout lateral movement.

It will be understood that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

I claim as my invention:

1. A mold for the production of cast hollow articles including, a shellhaving a main cavity corresponding substantially in shape to the articleto be cast, and a core positioned within the cavity correspondingsubstantially in shape to the opening within the cast article, a fillingspace in the mold at one end of, and forming a continuation of the maincavity, the end of the core extending beyond the filling space and beingpositioned in and supported by the mold, a cavity extension at the otherend of the main cavity, and supporting elements extending laterally fromthe mold into contact with the adjacent end portion of the core, thelatter end portion being located within said cavity extension, the endof the cavity extension being open to be positioned on a chill plate,and the cavity extension providing a growth zone in the casting which isoutside the dimensions of the finished article.

2. A mold as in claim 1 in which the supporting elements are located inthe growth zone.

3. A mold for the production of cast hollow articles which aredirectionally solidified by casting in a heated mold with one end of thecasting against a chill plate, said mold including a shell having a maincavity corresponding in shape to the article, a filling cavity formingan extension at one end of the main cavity, and a growth zone cavityextension at the opposite end of the main cavity, the growth zone cavityextension having an open end for contact with the chill plate, and acore positioned within the cavities and extending from the fillingcavity through the main cavity and into the growth zone to define aspace within the cast article, said core extending into and beingsupported by the portion of the mold defining the filling cavity, theother end of the core terminating within the growth zone cavityextension (short of the open end to be out of contact with the chillplate), said mold in this growth zone extension having inwardlyextending elements to engage and support the end of the core to locateit within the growth zone.

4. A mold as in claim 3 in which the elements are located externally ofthe part of the cast article that is used as a finished part.

5; A mold as in claim 3 in which the lateral dimension of the extensionsis such as not to interfere with the desired grain growth in the castarticle.

6. A mold as in claim 3 in which the temperature of the mold and thechill plate is so controlled as to produce directional grain growth fromthe growth zone to the filling cavity, and the core supporting elementsare so positioned and have such a lateral dimension as not to affectdetrimentally the directional grain growth within the casting.

1. A mold for the production of cast hollow articles including, a shellhaving a main cavity corresponding substantially in shape to the articleto be cast, and a core positioned within the cavity correspondingsubstantially in shape to the opening within the cast article, a fillingspace in the mold at one end of, and forming a continuation of the maincavity, the end of the core extending beyond the filling space and beingpositioned in and supported by the mold, a cavity extension at the otherend of the main cavity, and supporting elements extending laterally fromthe mold into contact with the adjacent end portion of the core, thelatter end portion being located within said cavity extension, the endof the cavity extension being open to be positioned on a chill plate,and the cavity extension providing a growth zone in the casting which isoutside the dimensions of the finished article.
 2. A mold as in claim 1in which the supporting elements are located in the growth zone.
 3. Amold for the production of cast hollow articles which are directionallysolidified by casting in a heated mold with one end of the castingagainst a chill plate, said mold including a shell having a main cavitycorresponding in shape to the article, a filling cavity forming anextension at one end of the main cavity, and a growth zone cavityextension at the opposite end of the main cavity, the growth zone cavityextension having an open end for contact with the chill plate, and acore positioned within the cavities and extending from the fillingcavity through the main cavity and into the growth zone to define aspace within the cast article, said core extending into and beingsupported by the portion of the mold defining the filling cavity, theother end of the core terminating within the growth zone cavityextension (short of the open end to be out of contact with the chillplate), said mold in this growth zone extension having inwardlyextending elements to engage and support the end of the core to locateit within the growth zone.
 4. A mold as in claim 3 in which the elementsare located externally of the part of the cast article that is used as afinished part.
 5. A mold as in claim 3 in which the lateral dimension ofthe extensions is such as not to interfere with the desired grain growthin the cast article.
 6. A mold as in claim 3 in which the temperature ofthe mold and the chill plate is so controlled as to produce directionalgrain growth from the growth zone to the filling cavity, and the coresupporting elements are so positioned and have such a lateral dimensionas not to affect detrimentally the directional grain growth within thecasting.